1. Field of the Invention
The present invention relates generally to a technique of controlling a light-emitting unit including light-emitting devices designed to send forth an optical information beam toward a target object, and, more particularly, to an optical information recording equipment including a light-source unit for providing a write beam representing information to an optical information storage medium. The invention also relates to a laser control system for use in an optical record and read/write apparatus employing a round disk-shaped photosensitive record carrier body detachably set therein.
2. Description of the Related Art
With the recent advances in the technology of semiconductor lasers as solid-state light-emitting elements, optical information recording/reproducing apparatuses including an optical head device using output laser light as a write beam or read beam are increasing in applicability. Typically, an optical head device includes a semiconductor laser as its light source. The optical head device also includes a driver circuitry for adjusting a bias current of the laser such that the output light of the laser is at a power level preselected in accordance with an operation mode, and an optical system for projecting the adjusted laser light onto a target recording medium in an in-focus state. The above-described constituent elements are packed into a small-sized housing at a high density.
As one of optical information record/reproduction apparatuses, digital audio information record/ reproduction apparatuses are commercially important. An apparatus of this type is expected to be also used as a peripheral device such as an external storage unit of a computer system in the near future. As a digital audio information record/reproduction apparatus, there is known a recording/reproducing apparatus suitably used to record or play back information on or from a prerecorded optically accessible information-storage medium. Typically, a round disk-shaped record carrier body is rotatably set, as the above-mentioned recording medium, on the optical read apparatus. An optical head device is movably arranged in the radial direction of the rotating disk carrier body, and focuses a power-controlled read or write beam onto an optically detectable radiation sensitive layer of the disk carrier body. To do this, the device includes a forward optical path (light-transmitting system) for guiding a beam output from the laser to the disk carrier, and a reverse optical path (light-receiving system) for guiding light reflected by the disk carrier to a photodetector. Obviously, each of these light-transmitting and light-receiving systems is constituted by independent and distinct optical components. The photodetector performs photoelectric conversion to generate an electrical reproduction signal indicative of the read information.
As the demand for a decrease in the size of an optical recording/reproducing apparatus becomes stronger, an optical head device incorporated therein needs to be decreased in size. In order to decrease the size of the optical head device, it is especially important to reduce the number of components required, especially, those of light-transmitting and light-receiving optical system components. In order to meet this demand, attempts have been made to use optical components serving as both light-transmitting and light-receiving components or share optical components between the two systems.
Unfortunately, there is a trade-off in sharing optical components between the light-transmitting and light-receiving systems. If the transmission efficiency (influencing the light-use ratio) is improved and the imaging magnification is reduced, as inherently required in the light-transmitting system, the performance of the light-transmitting system can be improved. In this case, however, the signal detection stability required for the light-receiving system is impaired, and the performance of the light-receiving system, which is inherently required to have a large imaging magnification, is degraded. Another way of saying this is that there is a decisive difference in the imaging magnification required between the light-transmitting and light-receiving systems. That is, the former is required to have a small imaging magnification, but the latter is required to have a large imaging magnification. These conflicting conditions pose a serious bar against sharing of optical components for a decrease in size. Therefore, if a conventional compact optical head device exclusively used for reproduction is simply designed to be also used as a recording optical device, the light-use ratio is decreased. In order to compensate for this, a laser having higher power must inevitably be used. This may undesirably lead to an increase in power consumption, an increase in size, and an increase in manufacturing cost.